The United States has completed a test of "Grand Alliance-Advanced Television (GA-ATV)" as a digital television transfer system, which is a new television standard capable of replacing the NTSC (National Television System Committee) standard. The GA-ATV system (so-called "GA-HDTV" or "GA-VSB" system), as standardized by the Advanced Television System Committee (ATSC), adopts a vestigial side band (VSB) modulation method as a digital transfer method. Specifically, an 8-VSB using 8 levels is utilized for a terrestrial broadcast mode and a 16-VSB using 16 levels is utilized for a high speed cable mode.
A new ATV signal is transferred together with a conventional analog television signal (NTSC signal) via a television channel which is not in use in a given geographic region ("taboo" channel). Additionally, the new ATV signal may use the same channel as that of an adjacent analog television signal. Accordingly, a GA-VSB receiver must be resistant NTSC co-channel interference. Therefore, when a co-channel and a co-broadcast in which the HDTV signal transfer band of the GA-VSB mode and the NTSC signal transfer band of the current broadcast mode are the same, a method for removing a carrier, in which the energy of a signal is concentrated, is usually used in order to remove the co-channel interference. For example, a comb filter may be used as a representative method. A co-channel interference canceler including a comb filter, and the frequency characteristics of the comb filter, are shown in FIGS. 1 and 2A through 2D, as disclosed in "Guide to the use of the digital television standard for HDTV transmission", pp. 104.about.107, Doc.A/54, submitted to the United State Advanced Television System Committee, Apr. 12, 1995.
As shown in FIG. 1, a conventional co-channel interference canceler generally includes of a co-channel interference rejection filter (NRF; NTSC rejection filter) 110, a driving signal generator and a selector (MUX) 130. Here, the driving signal generator includes all elements from a subtractor 120 to a minimum energy detector 129.
When an NTSC signal exists in an HDTV channel, the NTSC signal acts as an interference to the HDTV signal. Therefore, since the NTSC signal has a predetermined carrier frequency offset (about 0.89 MHz) with respect to the HDTV signal in a co-channel situation where the HDTV signal and the NTSC signal are simultaneously broadcast, the NRF 110 provides the same effect as if the NTSC signal was modulated by a frequency offset with respect to the HDTV signal in a base band region. In the case of the NTSC signal, all energy is concentrated on an original DC component such as a modulation carrier. Thus, a subtractor 112 in the NRF 110 subtracts a symbol delayed by a 12-symbol delay (12D) 111 from an input symbol (received I channel data symbol) to remove the modulation carrier component, thereby decreasing the effect of the NTSC signal. In the GA-VSB method, a co-channel interference signal is removed using a comb filter having a 12-symbol delay in consideration of the relationship with a precoder of a transmitter.
On the other hand, a comb filter of 6 MHz VSB signal band, as shown in FIG. 2A, has 6 frequency nulls, as shown in FIG. 2B. The NTSC signal, which interferes with the HDTV signal, concentrates its energy on a visual carrier, a chrominance subcarrier and an aural carrier. However, since these carriers are located near frequency notches of the comb filter, the energy of the NTSC signal passed through the comb filter decreases significantly. Thus, the comb filter is excellent in removing the NTSC interference signal and has a simple structure. FIGS. 2C and 2D are diagrams showing band edges for the whole channel in detail.
Although, the comb filter proposed by the ATSC standard is excellent at removing the NTSC interference signal, the structure of the comb filter is characterized by the subtraction between two signals having a full gain, so that the output of the comb filter is increased from an 8-level (.+-.1, .+-.3, .+-.5, .+-.7) signal to a 15-level (0, .+-.2, .+-.4, .+-.6, .+-.8, .+-.10, .+-.12, .+-.14) signal, and simultaneously the power of the additive Gaussian noise is increased by 3 dB. This results in a 3 dB loss of signal-to-noise ratio (SNR) before and after the comb filter. Further, the performance of a receiving system is degraded by the drop in SNR.
On the other hand, the driving signal generator compares a previously stored field sync reference pattern with a field sync of data symbols being received to determine whether the NTSC signal is mixed with the HDTV signal using the accumulated value of the power (energy) of the difference between the field sync reference pattern and the field sync. Here, the received VSB data frame includes two fields, and a field sync segment, which is the first segment of each field, having a field sync sequence representing the start of the field. Since the field sync sequence has a predetermined shape, the field sync signal is used as a known signal.
The driving signal generator includes a first path portion (120.about.122), a second path portion (123.about.128), and a minimum energy detector 129. The first path portion (120.about.122) compares the reference signal (field sync reference pattern) with the known signal (field sync), without passing through the comb filters 110 and 123, to calculate the accumulated value of the power (energy) of the compared difference. The second path (123.about.128) compares the received filter sync, filtered by the comb filter 110, with the field sync reference pattern filtered by the comb filter 123, to calculate the accumulated value of power (energy) of the compared difference. The minimum energy detector 129 selects the output of the path portion having less energy, and generates a driving signal NRF.sub.-- sel for determining whether the NTSC signal is mixed with the HDTV signal. That is, if an NTSC signal is mixed therewith, the energy of the second path passed through the comb filters 110 and 123 is less. Otherwise, the energy of the first path portion which does not pass the comb filters 110 and 123 is less. The driving signal NRF.sub.-- sel represents whether or not the NTSC signal is included such that the driving signal NRF.sub.-- sel is a logic "1" if the NTSC signal exists and a logic "0" if the NTSC signal does not exist. The selector 130 selects the received data symbol or the data symbol passed through the comb filter 110, according to the driving signal NRF.sub.-- sel.
The conventional co-channel interference canceler shown in FIG. 1 selects the result of operating the NRF for removing the co-channel interference, using the energy of the error between the received known signal and the reference signal. However, this method requires a multiplier for calculating the energy, thereby resulting complicated hardware and unreliable decision ability. That is, when an equalizer or an error corrector is attached after the co-channel interference canceler of a general receiver, a real decision error rate is used as a more important basis for the decision than the energy of error. This is because the process performed after the co-channel interference canceler relates to the decision value having a closer relationship with the decision error.
As described above, the comb filter which is used as the co-channel interference rejection filter can remove the co-channel interference. However, other interference sources such as ghost may cause the comb filter to operate abnormally, even though co-channel interference does not exist. The performance is attenuated by about 3 dB when the comb filter operates for noise removal. Thus, for optimizing the performance of the system, the comb filter must operate only when the co-channel interference exists. Therefore, it is necessary to determine precisely whether or not the co-channel interference exists according to the channel condition.
As a conventional co-channel interference canceler, U.S. Pat. No. 5,546,132 discloses a detector for detecting the NTSC co-channel interference during the entire period of the received data without using the data field sync reference pattern. Additionally, U.S. Pat. No. 5,594,496 discloses a detector for generating a subtraction signal by comb-filtering a received signal including a field sync of successive fields, and comparing the subtraction signal passed through the comb filter with a subtraction signal which does not pass the comb filter, to remove the NTSC co-channel interference and other interferences according to the result of the comparison.